When babies crawl, their movement across floors, especially on carpets, kicks up high levels of dirt, skin cells, bacteria, pollen, and fungal spores, according to a new study.

Crawling babies inhale a dose of those bio bits into their lungs that is four times (per kilogram of body mass) what an adult would breathe walking across the same floor.

“Such exposures act to stimulate and challenge your immune system…”

As alarming as that sounds, lead researcher Brandon Boor, assistant professor of civil engineering and environmental & ecological engineering at Purdue University, is quick to add that it isn’t necessarily a bad thing,

“We are interested in the biological material an infant inhales, especially during their first year of life when they are crawling,” says Boor.

“Many studies have shown that inhalation exposure to microbes and allergen-carrying particles in that portion of life plays a significant role in both the development of, and protection from, asthma and allergic diseases. There are studies that have shown that being exposed to a high diversity and concentration of biological materials may reduce the prevalence of asthma and allergies later in life,” he explains.

Previous studies have determined how much dirt and biological material is kicked up and resuspended into the air when an adult walks indoors, but this is the first study to look at what happens with infants and their unique forms of locomotion.

Rolling, sliding, and crawling

Human babies are the only mammals that can’t get up and walk soon after being born. Elephants, giraffes, and horses can all take a few wobbly steps soon after they enter the world, but it’s months before a human can claim the same accomplishment.

As babies roll, slide, and crawl on the floor, their movements stir up particulates into the air, and their mouths and nostrils are much closer to the floor where the concentrations are greater. This is countered somewhat by the fact that babies tend to move in much shorter bursts of activity than do older children or adults.

To study just how much of the floor debris babies breathe, researchers built a robotic crawling baby (which is much less adorable than the real thing) and tested it crawling on actual carpet samples they had removed from homes. Then they measured and analyzed the particulates in the breathing zone.

(Credit: Purdue)

“We used state-of-the-art aerosol instrumentation to track the biological particles floating in the air around the infant in real-time, second by second. The instrument uses lasers to cause biological material to fluoresce,” Boor says.

“Most bacterial cells, fungal spores, and pollen particles are fluorescent, so they can be reliably distinguished from non-biological material in the air. We also worked with a microbiology group at Finland’s National Institute for Health and Welfare, which conducted DNA-based analysis of the microbes we collected onto filters.”

The researchers found that a concentrated cloud of resuspended particles forms around the babies and that the concentrations around them can be as much as 20 times greater than the levels of material higher in the room.

Hygiene hypothesis

Further, infants’ bodies aren’t as good at blocking this dust storm, Boor says.

“For an adult, a significant portion of the biological particles are removed in the upper respiratory system, in the nostrils and throat. But for very young children, they more often breathe through their mouths, and a significant fraction is deposited in the lower airways—the tracheobronchial and pulmonary regions. The particles make it to the deepest regions of their lungs,” he explains.

In the late 1980s, British epidemiologist David Strachan was the first to propose the “hygiene hypothesis,” which says that too clean of an environment may suppress the development of the immune system. Allergists also sometimes refer to this as “the farming effect.”

“Exposure to certain bacterial and fungal species can result in the development of asthma, but numerous studies have shown that when an infant is exposed to a very high diversity of microbes, at a high concentration, they can have a lower rate of asthma later in life. Such exposures act to stimulate and challenge your immune system,” Boor says.

In Western societies, infants spend nearly all of their time indoors, where dust resuspension may contribute significantly to their respiratory encounters with biological material.

“While our research established new methods for infant microbial exposure assessment, much remains to be discovered,” Boor says.